Fluoro-chloro-benzene derivatives

ABSTRACT

Disclosed are compounds of formula I  &lt;IMAGE&gt;  wherein R is C1-12-alkyl or halogenoalkyl wherein one or two non-adjacent CH2 groups are optionally replaced by -O-, A is   &lt;IMAGE&gt; (2)  Q is a bivalent radical of formula (2), and L0, L1 and L2 are each independently H or F, with the proviso that at least one of L0, L1 and L2 present in the molecule denotes F.

This application is a continuation of U.S. patent application Ser. No.08/264,108, filed Jun. 22, 1994, abandoned, which a divisionalapplication of U.S. patent application Ser. No. 07/902,801, filed Jun.24, 1992, now U.S. Pat. No. 5,482,653, which is a continuation of Ser.No. 08/572,976 filed Jul. 31, 1990, abandoned which is based on GreatBritain application No. 8912339, filed May 30, 1990, United Kingdomapplication No. 8913441, filed Jun. 12, 1989, and Great Britainapplication No. 8913442, filed Jun. 12, 1989.

The invention relates to fluoro-chloro-benzene derivatives of theformula I

Fluoro-chloro-benzene derivatives of the formula I ##STR3## wherein Rdenotes an alkyl or halogenoalkyl residue of up to 12 C atoms whereinone or two non-adjacent CH₂ groups may also be replaced by --O-- and/or--HC═CH--,

the radical A is a ring of the formula (1) or (2): ##STR4## Q is abivalent radical of the formula (1) or (2) or a single bond or--ifradical A denotes trans-1,4-cyclohexylene--is also a bivalent radical ofthe formulae (3), (4) or (5): ##STR5## and L⁰, L¹ and L² are eachindependently H or F, with the proviso that at least one of L⁰, L¹ andL² present in the molecule denotes F,

and also to liquid crystalline media being a mixture of at least 2compounds, characterized in that at least one compound is a fluorinatedbenzene derivative according to formula I.

The invention was based on the object of discovering new stable liquidcrystal or mesogenic compounds which are suitable as components ofliquid crystalline media and, in particular, have advantageous valuesfor optical and dielectric anisotropy combined with low viscosity andhigh nematogenity.

Similar chloro-benzene derivatives without lateral fluorine substitutionare described in DE 31 36 624, DE 31 39 130, JP 57/183727 and JP59/81375 and also by S. M. Kelly and Hp. Schad, Helv. Chim. Acta 68 (5),1444-52 (1985) and H. Takatsu et al., Mol. Cryst. Liq. Cryst. 100 (3-4),345-55 (1983).

It has now been found that laterally fluorinated compounds of formula Iare highly suitable as polar components of liquid crystalline media. Inparticular, they have especially advantageous values of optical anddielectric anisotropy and are not strongly smectogenic. It is alsopossible to obtain stable liquid crystal phases with a broad nematicmesophase range including a good deep temperature behaviour, a highresistivity and a comparatively low viscosity with the aid of thesecompounds.

Depending on the choice of R, A, Q, L¹ and L² the compounds of theformula I can be used as the base materials from which liquid crystalmedia are predominantly composed; however, it is also possible forcompounds of the formula I to be added to liquid crystal base materialsof other classes of compounds, for example in order to influence thedielectric and/or optical anisotropy and/or the viscosity and/or thenematic mesophase range of such a dielectric.

The compounds of the formula I are colourless in the pure state and areliquid crystalline in a temperature range which is favourably placed forelectrooptical use. They are very stable towards chemicals, heat andlight.

The invention thus relates to the benzene derivatives of the formula I,to liquid crystalline media with at least two liquid crystallinecomponents, wherein at least on component is a compound of the formula Iand to liquid crystal display devices containing such media.

Above and below, R, A, Q, L¹ and L² have the meaning given unlessexpressly indicated otherwise.

The compounds of the formula I include benzene derivatives of theformulae Ia to Ig: ##STR6##

In formula Ib Q is preferably trans-1,4-cyclohexylene or 1,4-phenylene.In formula Ie Q is a bivalent radical of formula (3), (4) or (5).

R is preferably alkyl, alkoxy, oxaalkyl or alkenyl and can exhibit astraight-chain or branched structure.

Alkyl or alkoxy preferably are straight-chain and have 2, 3, 4, 5, 6 or7 C atoms. Accordingly they are preferably ethyl, propyl, buryl, pentyl,beryl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy or heptoxy, alsomethyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl,pentadecyl, methoxy, octoxy, nonoxy, decoxy, undecoxy, dodecoxy,tridecoxy or tetradecoxy.

Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl),2-(=ethoxymethyl) or 3-oxybutyl (=2-methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4- 5- or 6-oxaheptyl, 2-,3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl or2-, 3-, 4-, 5-, 6-, 7-, 8-, or 9-oxadecyl.

Alkenyl is preferably straight-chain and has 2 or 10 C atoms. It isaccordingly, in particular, vinyl, prop-1- or prop-2-enyl, but-1-, -2-or -3-enyl, pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or-5-enyl, hept-1-, -2-, -3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-,-5- -6- or -7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl ordec-1-, -2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl.

Halogenoalkyl is preferably an alkyl residue (perferablystraight-chained) carrying a fluoro or chloro substituent on one of theup to 12 carbon atoms.

Compounds of the formula I containing a branched terminal group canoccasionally be of importance because of an improved solubility in thecustomary liquid crystal base materials, but in particular as chiraldoping substances if they are optically active.

Branched groups of this type as a rule contain not more than one chainbranching. Preferred branched radicals are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl, 2-methylbutyl, isopentyl,(=3-methylbutyl), 2-methylpentyl, 2-ethylhexyl, 2-propylpentyl, 2-octyl,isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy,2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 2-methylhexoxy,1-methylhexoxy, 1-methylheptoxy (=2-octyloxy), 2-oxa-3-methylbutyl,3-oxa-4-methylpentyl, 4-methylhexyl, 2-nonyl, 2-decyl, 2-dodecyl,6-methyloctoxy, oxy, 2-methyl-3-oxapentyl and 2-methyl-3-oxahexyl.

In the case of compounds with a branched terminal group R, formula Iincludes both the optical antipodes and racemates as well as mixturesthereof.

Preferably only one of the substituent L⁰, L² and L² present in themolecule deno tes F. Furthermore preferred are those compounds of theformula I wherein ##STR7##

Of the compounds of the formula I and subformulae, thereof, those inwhich at least one of the radicals contained therein has one of thepreferred meanings given are preferred.

The compounds of the formula I are prepared by methods which are knownper se, such as are described in the literature (for example in thestandard works, such as Houben-Weyl, Methoden der Organischen ChemieMethods of Organic Chemistry, Georg Thieme Verlag, Stuttgart), and inparticular under reaction conditions which are known and suitable forthe reactions mentioned. Variants which are known per se and are notmentioned in more detail here can also be used in this connection.

If desired, the starting materials can also be formed in situ, such thatthey are not isolated from the reaction mixture but are immediatelyreacted further to give the compounds of the formula I.

Preferred routes for preparation are shown in the following schemes:##STR8##

In schemes 1 and 2 ring B denotes a ring of formula (2) and m is 0 or 1.##STR9##

In scheme 3 ring A denotes trans-1,4-cyclohexylene or 1,4-phenylene.trans-1,4-Cyclohexylene is preferred. ##STR10##

All starting materials are known or can be prepared in analogy to knowncompounds. The alkenes can be made by Heck reaction ofcyclohexylbromides and vinyl bromide. The starting acids can be obtainedfrom the known cyano compounds.

4-chloro-2-fluorobromobenzene can be made by diazotisation ad Sandmeyerreaction on 4-chloro-2-fluoroaniline. 4-Bromo-2-fluoroalkylbenzenes aremade by the following route: ##STR11##

The 2-fluoro-4-bromo-3-L² -4'-chloro-biphenyls (L² =H or F), forexample, can be made by transition metal catalyzed cross-couplingreactions (E. Poetsch, Kontakte (Darmstadt) 1988 (2) p. 15): ##STR12##

Other routes are apparent to the skilled worker. All these steps and thecorresponding reaction conditions are known to the skilled worker.

In addition to one or more compounds for formula I the liquid crystalmedia according to the invention preferably contain 2-40 components andin particular 4-30 components. Liquid crystal media being composed ofone or more compounds of formula I and 7-25 other components areespecially preferred.

These additional components are preferably chosen from the nematic ornematogenic (monotropic or isotropic) substances; in particular from theclasses of azoxybenzenes, benzylideneanilines, biphenyls, terphenyls,phenyl or cyclohexyl benzoates, phenyl or cyclohexylcyclohexanecarboxylates, phenyl or cyclohexyl cyclohexylbenzoates,phenyl or cyclohexyl cyclohexylcyclohexanecarboxylates,cyclohexylphenylbenzoates, cyclohexylphenyl cyclohexanecarboxylates,cyclohexylphenyl cyclohexylcyclohexanecarboxylates, phenylcyclohexanes,cyclohexylbiphenyls, phenylcyclohexylcyclohexanes,cyclohexylcyclohexanes, cyclohexylcyclohexenes,cyclohexylcyclohexylcyclohexene, 1,4-bis-cyclohexylbenzenes,4,4'-bis-cyclohexylbiphenyls, phenyl- or cyclohexylpyrimidines, phenyl-or cyclohexylpyridines, phenyl- or cyclohexyldioxanes, phenyl- orcyclohexyl-1,3-dithianes, 1,2-diphenylethanes, 1,2-dicyclohexylethanes,1-phenyl-2-cyclohexylethanes,1-cyclohexyl-2-(4-phenyl-cyclohexyl)-ethanes,1-cyclohexyl-2-biphenylethanes, 1-phenyl-2-cyclohexyl-phenylethanes,optionally halogenated stilbenes, benzyl phenyl ethers, tolanes andsubstituted cinnamic acids.

The 1,4-phenylene groups of these compounds may be fluorinated.

The most important compounds which are possible constituents of liquidcrystal media according to the invention can be characterized by theformalae 1, 2, 3, 4 and 5: ##STR13##

In the formulae 1, 2, 3, 4 and 5 L and U may be equal or different fromeach other. L and U independently from each other denote a bivalentresidue selected from the group consisting of -Phe-, -Cyc-, -Phe-Phe-,-Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, -G-Phe-, -G-Cyc- and their mirrorimages; in this compilation of residues Phe denotes unsubstituted orfluorinated 1,4-phenylen, Cyc trans-1,4-cyclohexylene or1,4-cyclohexenylen, Pyr pyrimidine-2,5-diyl or pyridine-2,5-diyl, Dio1,3-dioxane-2,5-diyl and G 2-(trans-1,4-cyclohexyl)-ethyl,pyrimidine-2,5-diyl, pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl.

One of the residues L and U is preferably Cyc, Phe or Pyr. U preferablydenotes Cyc, Phe or Phe-Cyc. The liquid crystal media according to theinvention preferably contain one or more components selected from thecompounds of formulae 1, 2, 3, 4 and 5 with L and U meaning Cyc, Phe andPyr, said liquid crystal media further containing at the same time oneore more components selected from the compounds of formulae 1, 2, 3, 4and 5 with one of the residues L and U denoting Cyc, Phe and Pyr and theother residue being selected from the group consisting of -Phe-Phe-,-Phe-Cyc-, -Cyc-Cyc-, -G-Cyc-, said liquid crystal media containing inaddition to this optionally one or more components selected from thecompounds of formulae 1, 2, 3, 4 and 5 with L and U being selected fromthe group consisting of -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc.

In a preferred subgroup of the compounds of formulae 1, 2, 3, 4 and 5(subgroup 1) R' and R" are independently from each other alkyl, alkenyl,alkoxy, alkenoxy with up to 8 carbon atoms. R' and R" differ from oneanother in most of these compounds, one of the residues usually beingalkyl or alkenyl. In another preferred subgroup of the compounds offormulae 1, 2, 3, 4 and 5 (subgroup 2) R" denotes --CN, --CF₃, --F, --Clor --NCS while R' has the meaning indicated in subgroup 1 and ispreferably alkyl or alkenyl. Other variants of the envisagedsubstituents in the compounds of formulae 1, 2, 3, 4 and 5 are alsocustomary. Many such substances are commercially available. All thesesubstances are obtainable by methods which are known from the literatureor by analogous methods.

The liquid crystal media according to the invention preferably containin addition to components selected from subgroup 1 also components ofsubgroup 2, the percentage of these components being as follows:

subgroup 1: 20 to 90%, in particular 30 to 90%

subgroup 2: 10 to 50%, in particular 10 to 50%.

In these liquid crystal media the percentages of the compounds accordingto the invention and the compounds of subgroup 1 and 2 may add up togive 100%.

The media according to the invention preferably contain 1 to 40%, inparticular 5 to 30% of the compounds according to the invention. Mediacontaining more than 40%, in particular 45 to 90% of the compoundsaccording to the invention are further preferred. The media containpreferably 3, 4 or 5 compounds according to the invention.

The media according to the invention are prepared in a manner which iscustomary per se. As a rule, the components are dissolved in oneanother, advantageously at elevated temperature. The liquid crystalmedia according to the invention can be modified by suitable additivesso that they can be used in all the types of liquid crystal displaydevices. Such additives are known to the expert and are described indetail in the literature (H. Kelker/R. Hatz, Handbook of LiquidCrystals, Verlag Chemie, Weinheim, 1980).

For example, it is possible to add pleochroic dyestuffs to preparecolored guest-host systems or substances for modifying the dielectricanisotropy, the viscosity and/or the orientation of the nematic phases.

The following examples are to be construed as merely illustrative andnot limitative. m.p.=melting point, c.p.=clearing point. In theforegoing and in the following all parts and percentages are by weightand the temperatures are set forth in degrees Celsius. "Customarywork-up" means that water is added, the mixture is extracted withmethylene chloride, the organic phase is seperated off, dried andevaporated, and the product is purified by crystallization and/orchromatography.

Further are:

C: crystalline-solid state, S: smectic phase (the index denoting the typof smectic phase), N: nematic phase, Ch: cholesteric phase, I: isotropicphase. The number being embraced by 2 of these symbols denotes thetemperature of phase change.

EXAMPLE 1

A mixture of 0.1 mole of 4'-n-propyl-4-biphenylboronic acid (obtained bythe reaction of the Grignard reagent of 4'-n-propyl-4-bromobiphenyl andtrimethyl borate), 0.1 mole of 4-chloro-3-fluorobromobenzene, tetrakis(triphenylphosphine) palladium (1 mole %), 2 m sodium carbonate solution(150 ml), toluene (250 ml) and IMS (60 ml) is stirred and refluxed for16 hrs under a nitrogen atomosphere. After usual work-up4-chloro-3-fluoro-4'-(p-n-propylphenyl)-biphenyl is obtained.

EXAMPLES 2 TO 11

In place of the 4'-n-propyl-4-bromobiphenyl the other well knownrelevant bromo compounds can be used for preparation of the followingcompounds in analogy to example 1:

    ______________________________________                                         ##STR14##                                                                    R          X                                                                  ______________________________________                                        (2)  ethyl                                                                                    ##STR15##                                                     (3)  n-pentyl                                                                                 ##STR16##   C 171 S.sub.A 178 N 194 I                         (4)  n-propyl                                                                                 ##STR17##   C 108 N 143.5 I, Δε = 6.5           (5)  n-pentyl                                                                                 ##STR18##                                                     (6)  ethyl                                                                                    ##STR19##                                                     (7)  n-propyl                                                                                 ##STR20##                                                     (8)  n-butyl                                                                                  ##STR21##                                                     (9)  n-pentyl                                                                                 ##STR22##                                                     (10) n-heptyl                                                                                 ##STR23##                                                     (11) n-nonyl                                                                                  ##STR24##                                                     ______________________________________                                    

EXAMPLE 12

Under the condition described in example 1 0.1 mole of4-bromo-2-fluoro-n-pentylbenzene is coupled with 4-chlorophenylboronicacid (made from 4-chlorobromobenzene via the Grignard reagent andtrimethyl borate). After usual work-up4-chloro-2'-fluoro-4'-n-pentylbiphenyl is obtained.

EXAMPLES 13 TO 16

The following compounds are prepared analogously:

(13) 4-chloro-2'-fluoro-4'-n-propylbiphenyl

(14) 4-chloro-2'-fluoro-4'-n-hexanoyloxybiphenyl, K24 I

(15) 4-chloro-2'-fluoro-4'-n-heptylbiphenyl

(16) 4-chloro-2'-fluoro-4'-n-nonylbiphenyl

EXAMPLE 17

A solution of 4-n-propylcyclohexanone (0.1 m) in THF is slowly added toa warm solution of 4-chloro-3-fluorophenyl magnesium bromide (0.11 m)(made from 4-chloro-3-fluorobromobenzene) in THF under nitrogen. Theresulting alcohol is dehydrated by sulphuric acid.

Hydrogenation of the resulting olefin over Pd/C gives a mixture ofcis/trans isomers.4-(Trans-4-n-propylcyclohexyl)-2-fluoro-1-chlorobenzene is obtained bychromatography and crystallization, C -15 I, Δε=5.2.

EXAMPLES 18 TO 20

The following compounds are prepared analogously:

(18) 4-(trans-4-n-butylcyclohexyl)-2-fluoro-1-chlorobenzene

(19) 4-(trans-4-n-pentylcyclohexyl)-2-fluoro-1-chlorobenzene, C -3 I,Δε=4.8

(20) 4-(trans-4-n-heptylcyclohexyl)-2-fluoro-1-chlorobenzene

EXAMPLES 21 TO 23

The following compounds are obtained analogously by using4-(trans-4-alkylcyclohexyl)-cyclohexanones as educts:

(21) 4-(trans,trans-4'-ethylbicyclohexyl-4-yl)-2-fluoro-1-chlorobenzene

(22)4-(trans,trans-4'-n-propylbicyclohexyl-4-yl)-2-fluoro-1-chlorobenzene, C42 N 157.3 I, Δε-7.0

(23)4-(trans,trans-4'-n-pentylbicyclohexyl-4-yl)-2-fluoro-1-chlorobenzene

EXAMPLE 24

The compound from example 17 is lithiated using butyl lithium at lowtemperatures by the usual procedure and then converted to a boronic acidby the addition of trimethyl borate at low temperatures by the usualprocedure. The boronic acid is the coupled with 4-bromo-chlorobenzeneaccording to example 1.

After usual work-up4-chloro-2'-fluoro-4'-(trans-4-n-propylcyclohexyl)-biphenyl is obtainedC 113 N 142.1 I.

EXAMPLES 25 TO 27

The following compounds are prepared analogously:

(25) 4-chloro-2'-fluoro-4'-(trans-4-n-butylcyclohexyl)biphenyl

(26) 4-chloro-2'-fluoro-4'-(trans-4-n-pentylcyclohexyl)biphenyl C 106 N145.9 I

(27) 4-chloro-2'-fluoro-4'-(trans-4-n-heptylcyclohexyl)biphenyl

EXAMPLE 28

A mixture of 0.1 mole of2-fluoro-4-(trans-4-n-propylcyclohexylethyl)-phenyl boronic acid(obtained using butyllithium and trimethylborate at low temperaturesaccording to scheme 4), 0.1 mole of 4-bromochlorobenzene,tetrakis(triphenylphosphine)palladium (1 mole %) 2 m sodium carbonatesolution (150 ml), toluene (250 ml) and IMS (60 ml) is stirred andrefluxed for 16 hrs under a nitrogen atmosphere. After cooling theorganic layer is separated, washed with wafer and the solvent evaporatedoff. The crude4-(trans-4-propylcyclohexylethyl)-2-fluoro-4'-chlorobiphenyl is purifiedby chromatography on silica and crystallisation C 72 N 120 I.

EXAMPLES 29 TO 33

The following compounds are obtained analogously: ##STR25##

EXAMPLE 34

From 1-bromo-4-(trans-4-n-propylcyclohexylethyl)-benzene and3-fluoro-4-chlorophenylboronic acid (obtained by treating the Grignardreagent of 3-fluoro-4-chloro-bromobenzene with trimethylborate at 20°C.) 4-(trans-4-propylcyclohexylethyl)-3'-fluoro-4'-chlorobiphenyl isobtained in analogy to example 1, K 48 S_(A) 51 N 131 I.

EXAMPLES 35 TO 39

The following compounds are obtained analogously: ##STR26##

EXAMPLE 40

After hydrogenation of1-[p-(trans-4-n-propylcyclohexyl)phenyl]-2-(3-fluoro-4-chlorophenyl)-ethene(obtained according to scheme 6) and customary work-up1-[p-(trans-4-n-propylcyclohexyl)-phenyl]-2-(3-fluoro-4-chlorophenyl)-ethaneis obtained C 66.3 I.

EXAMPLE 41

A mixture of 0.1 mole of1-chloro-2-fluoro-4-(trans-4-n-propylcyclohexylethyl)benzene (obtainedby the reaction of 4-n-propylcyclohexylmethylphosphonium iodide with4-chloro-3-fluorobenzaldehyde followed by hydrogenation), 0.1M4-chlorophenylboronic acid (obtained by the reaction of4-bromochlorobenzene with magnesium and trimethylborate),tetrakis(triphenylphosphine) palladium (1 mole %) 2 m sodium carbonatesolution (150 ml), toluene (250 ml) and IMS (60 ml) is stirred andrefluxed for 16 hrs under a nitrogen atmosphere. After cooling theorganic layer is separated, washed with water and the solvent evaporatedoff. The crude4-(trans-4-propylcyclohexylethyl)-2-fluoro4'-chlorobiphenyl is purifiedby chromatography on silica and crystallisation.

EXAMPLE 42

Step 1 and Step 2

Buryl lithium (1.6 m) (95 ml) was slowly added to trans1,4-propylcyclohexylmethyl phosphonium iodide (70 g) in THF (140 ml) at20° C. and stirred for 1 hr. 3-fluorobenzaldehyde (16.5 g) in THF (20ml) was added dropwise and stirred for 15 mins. Water (100 ml) was addedand the volatiles distilled off. Dichloromethane (200 ml) was added andthe organic layer separated, washed with dilute hydrogenperoxide andthen ammonium ferrous sulphate and water. Column chromatography gave thecis/trans alkene which was hydrogenated with Pd/C in THF (100 ml) togive the required ethane.

Step 3

Product from step 2 (17.8 g) dissolved in THF (30 ml), potassiumbutoxide (8 g) in THF (30 ml) and DMPU (9.5 g) were cooled to -110° C.Butyl lithium (1.6 m) (50 ml) was added dropwise over 40 min and stirredat -100° C. for 1 hr. Trimethyl borate (8.2 g) in THF (20 ml) was addedand the reaction mixture allowed to warm to 20° C. Normal work up gavethe required boronic acid.

Step 4

A mixture of boronic acid (1.9 g) from the previous step,4-bromo-chlorobenzene (1.6 g), toluene (20 ml), ethanol (5 ml),palladium tetra (triphenylphosphine) (0.1 g) and sodium carbonatesolution (10 ml) was stirred and heated at reflux for 5 hrs. The productwas isolated by extraction and column chromatography to give4-(trans-4-n-propylcyclohexylethyl)-2-fluoro-4'-chlorobiphenyl.

EXAMPLE 43

Step 1

The Grignard reagent of 4-bromo-chlorobenzene (75.9 g) was prepared byreaction with magnesium (8.3 g) in THF (515 ml), after cooling to 20°C., zinc chloride (315 ml of 1.0 m soln in diethyl ether) was added withcooling, excess magnesium was filtered off. This mixture was added to amixture of 4-bromo-3-fluoro-1-iodobenzene (86.2 g) in THF (150 ml)containing palladium tetra (triphenylphosphine) (1.0 g) and stirred for2 days under nitrogen. Normal workup and chromatography gave4-bromo-2-fluoro-4'-chlorobiphenyl.

Step 2

The product from step 1 (18.6 g) was converted to the Grignard reagentusing magnesium (1.4 g) and THF (70 ml). Trans-4-pentylcyclohexanone(8.7 g) in THF (20 ml) was slowly added and the mixture then heatedunder reflux for 1 hr. After the usual workup 22.5 g of viscous oil wasisolated, this was dehydrated using toluensulphonic acid (0.2 g) intoluene (100 ml) to give the alkene (Scheme 6). After hydrogenation ofthe double bond the trans-isomer was isolated by extraction and columnchromatography to give4-(trans-4-n-pentylcyclohexyl)-2-fluoro-4'-chlorobiphenyl.

EXAMPLE 44

A mixture of 0.1 mole of1-chloro-4-[2-(4-(trans-4-n-propylcyclohexyl)-phenyl)-ethyl]-benzene(obtained by the reaction of4-(trans-4-n-propylcyclohexyl)-benzylphosphonium iodide with4-chlorobenzaldehyde followed by hydrogenation), 0.1 mole4-chloro-3-fluorophenylboronic acid (obtained by the reaction of1-bromo-3-fluror-4-chlorobenzene with magnesium and trimethylborate),tetrakis (triphenylphosphine) palladium (1 mole %), 2 m sodium carbonatesolution (150 ml), toluene (250 ml) and IMS (60 ml) is stirred andrefluxed for 16 hrs. After cooling the organic layer is separated,washed with wafer and the solvent evaporated off.

The crude1-[4-(trans-4-n-propylcyclohexyl)-phenyl]-2-(4'-chloro-3'-fluorobiphenyl-4-yl)-ethaneis purified by chromatography on silica and crystallization, C 100 N 208I.

We claim:
 1. Fluoro-chloro-benzene derivatives of the formula I##STR27## wherein R denotes an alkyl or halogenoalkyl residue of up to12 C atoms wherein one or two non-adjacent CH₂ groups may also bereplaced by --O--,the radical A is a ring of the formula (2): ##STR28##Q is a bivalent radical of the formula (2), and L⁰, L¹ and L² are eachindependently H or F, with the proviso that at least one of L¹ and L²denotes F.
 2. A benzene derivative according to claim 1, having formulaIa ##STR29##
 3. A benzene derivative of claim 1, having formula Ib##STR30## and Q is 1,4-phenylene.
 4. A liquid crystalline mediumcomprising a mixture of at least two compounds, wherein at least onecompound is a benzene derivative accordingly to claim
 1. 5. A liquidcrystal display device comprising a liquid crystalline medium accordingto claim
 4. 6. Fluoro-chloro-benzene derivatives of the formula I##STR31## wherein R denotes an alkyl or halogenoalkyl residue of up to12 C atoms wherein one or two non-adjacent CH₂ groups may also bereplaced by --O--,the radical A is a ring of the formula (2): ##STR32##Q is a bivalent radical of the formula (2), and L¹ and L² are eachindependently H or F, with the proviso that at least one of L¹ and L²denotes F.